The last several years have seen an increasing emphasis being placed on the design of internal combustion engines to make them more compact, lighter, and yet with improved fuel economy, efficiency, and increased power output per cylinder. As the demands on modern engines increase, the piston assembly is subject to higher combustion chamber pressures and temperatures.
Several attempts have been made to overcome the effects of these higher combustion chamber pressures by altering the design of the piston member and its associated components, such as the piston pin and the retaining device, relative to the piston member. An example is disclosed in UK Patent Application No. GB 2225087 issued May 23, 1990 to applicant Metal Levy S A Industria Comerclo. The publication discloses an articulated piston assembly comprised of a head having two pin bosses integral with it, a skirt portion, and a wrist pin mounted in the pin boss holes in the skirt portion. The assembly further includes a restraining member to prevent lateral deformation of the pin bosses towards the cylinder liner thereby imparting a higher strength to the piston assembly. The lower ends of the pin bosses are split and are clamped to the pin by means of nuts and bolts.
Another example of a piston assembly is disclosed in UK Patent Application No. 0222359 published May 20, 1987 to applicant AE PLC. The publication discloses an articulated piston assembly comprised of a crown portion having pin bosses and a skirt portion. The two portions have a common pin bore axis characterized in that the skirt portion is connected to the crown portion by a pair of stud pins fixedly positioned in the crown portion to prevent axial movement of the pin. In the preferred embodiment, the skirt portion comprises a plastic material. The stud pins may also comprise a plastic material to reduce the reciprocating mass of the piston assembly. The stud pins are retained by rubbing in constant or intermittent contact with the associated cylinder or cylinder liner wall.
Another example of a piston assembly is disclosed in U.S. Pat. No. 5,144,844 issued Sep. 8, 1992 to E. L. Kelly. The patent discloses a piston assembly including a piston rod with a pin receiving opening at its upper end, a piston head having a transverse bore, and a piston pin. The piston pin is disposed through the pin receiving opening of the piston rod and the transverse bore of the piston head to connect the piston head to the piston rod. The piston assembly also includes a piston skirt which is connected to the piston head. The piston pin includes an axial bore and the piston skirt has a transverse bore. At least one piston skirt pin is disposed through the piston pin axial bore and the transverse bore of the piston skirt to connect the piston skirt to the piston pin. This connects the piston skirt to the piston head. A fastening device secures the piston skirt pin in position within the piston skirt and the piston pin. In one embodiment, the piston skirt pin extends completely through the piston pin axial bore and the fastening device includes a pair of snap rings. In another embodiment, two piston skirt pins are used. Each piston skirt pin is disposed in a respective end of the piston axial bore. The fastening device includes complimentary threads formed on one end of each piston skirt pin and in the piston pin axial bore. Alternatively, the fastening device includes forming the piston skirt pin with a slightly larger diameter than the piston pin axial bore. Each piston skirt pin is then press fit into the piston pin axial bore.
Another example of a piston assembly is disclosed in U.S. Pat. No. 5,289,758 issued Mar. 1, 1994 to W. G. Berlinger. The patent discloses a piston assembly comprised of a cylindrical main body having a pair of radially inset leg members extending from the main body which are spaced apart from one another. Each of the leg members has a bore position therein. A cylindrical wrist pin is positioned within the bores. The piston assembly is further comprised of a pair of wrist pin plugs, one of each being positioned at least partially within the inner surface at each end of the wrist pin. The wrist pin plugs include a head portion having a preestablished diameter being less than the preestablished diameter of the wrist pin outer surface. The wrist pin plugs further include a shank portion extending from the head portion. The shank portion of the plug has a diameter which provides for an interference fit between the shank and the wrist pin. As the plugs are forced into the wrist pin, the interference fit between the inner surface of the wrist pin and the outer surface of the plug causes a resistive force to be applied to the inner surface of the wrist pin and retain the plugs within the wrist pin. During operation of the piston assembly, the shank portion retains the wrist pin in proper alignment in the piston assembly. The wrist pin and the attached plugs are free to move axially within the radially inset leg members.
In each of the prior art piston assemblies the fastening device secures the piston skirt to the piston head by using threads, a interference fit or snap ring grooves. Each of these types of embodiments causes a stress concentration factor that results in a weaker piston pin. The present invention is directed to overcome one or more of the problems as set forth above.